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Y=B(ECw-Aw) solves for the percent yield (Y) of a species where B is the percent yield decrease per unit salinity increase , Aw is the "salinity threshhold," -the salinity concentration at which that species begins to show effects and ECw is the salinity of the irrigation water, expressed as electrical conductivity. B and Aw aze experimentally-determined values, unique to each plant species. ECw is the measured value of the irrigation water in question. Four species are used in considering "native hay," the crop grown in the irrigated alluvial valley floor of Trout Creek. These aze orchazdgrass, smooth brome, timothy, and redtop. Of these, experimental data exist for orchardgrass and timothy, and data are available for marchas brome, a species similar to smooth brome (Ippolito, 1992). For orchardgrass, the threshhold value (Aw) for affect of salinity is 3.0 mmho/cm; the slope (B) equating to % yield loss per mmho/cm, is 6.2. The value determined by the Colorado Division of Minerals and Geology for material damage is the loss of 3% of production. So the Maas and Hoffman equation can be solved for the "maximum Ecw" of irrigation water by substituting 97 for Y, representing the minimum production that is considered not material damage. Y= 100-B(ECw-Aw) 97=100-6.2(ECw-1.5) 2.51 =Ecw Converting tomicro-mhos, the limiting value for orchazdgrass is 2510 micro-mhos/cm2. Using the aforementioned equation to convert the conductivity value to total dissolved salts (TDS), TDS = 0.367 (Ecw) EE 1.107 TDS = 0.367 (2510) EE 1.107 TDS = 2128 mg/! Following the same procedure for the other two grasses for which empirical data exist, the limiting values are determined to be 475 mg/! for marchus brome and 467 mg/( TDS for timothy. So the calculations show that the absolute "limiting" concentration for surface irrigation of native hay in the alluvial valley floor is 467 mg/l total dissolved salts. In reality, one can assume the mix Edr~e Mine 28 Pennii Renewnl No. 3